1. Field of the Invention
The present invention relates generally to flow sensors and, more particularly, to self-normalizing flow sensors and methods of use thereof.
2. Description of Related Technology
Sensors have been used to measure flow rates in various medical, process, and industrial applications, ranging from portable ventilators supplying anesthetizing agents to large-scale processing plants in a chemical plant. In these applications, flow control is an inherent aspect of proper operation which is achieved in part by using flow sensors to measure the flow rate of a fluid within the flow system. In many flow systems, e.g., fuel gas flow systems containing a binary mixture of methanol and water, the chemical composition of the fluid may change frequently. Also, a flow system is often required to flow more than one fluid having different chemical and thermophysical properties. For example, in a semiconductor processing system that passes a N.sub.2 gas, the N.sub.2 gas may at times be replaced by a H.sub.2 or He gas, depending on the needs of the process; or in a natural gas metering system, the composition of the natural gas may change due to non-uniform concentration profiles of the gas.
Measuring the flow rates of fluids of differing chemical compositions, i.e., differing in density, thermal conductivity, specific heat, etc., requires calibrations of the flow sensor. Without recalibration, the flow sensor could produce accurate flow rate measurements for one fluid but not another. Typically, flow sensors are calibrated upon their initial operation and, as such, are calibrated to compute accurate flow rate values only for fluids with a particular, narrow range of chemical composition.
Known ways of re-calibrating a flow sensor, or providing a calibrated flow rate measurement, do exist. In some instances, customers provide flow sensor manufacturers with information on the composition of each fluid to be measured by the flow sensor. From this information, manufactures perform calibration tests and obtain data for use in making calibration functions, or look-up tables, from which the flow rate sensor can be calibrated. In other instances, previously installed flow sensors will be taken off-line so that a re-calibration for a new fluid can be performed. This process essentially reinitializes a flow sensor for accurate measurement with respect to fluids having differing chemical compositions and, therefore, is a costly and inconvenient option for customers.
An on-line calibration technique had been developed using a property sensor, i.e., a thermal sensor, to measure the specific heat and thermal conductivity properties of a fluid and a flow sensor to measure an uncalibrated flow rate. The specific heat and thermal conductivity (along with absolute temperature and Prandtl No.) are related to a flow rate correction factor, C.sub.V, by a known equation. Therefore, these property sensors, connected to a flow channel through a dead-end recessed cavity into which fluid enters principally by diffusion, measure values which must then be applied to expensive and time-consuming computational circuitry or microprocessors before the correction factor, C.sub.V, and subsequent calibrated flow signal can be produced.
The calibration correction factor, C.sub.V, is related to the calibrated flow rate by the following expression: V.sub.c =V.sub.u /C.sub.V, where V.sub.u is the uncalibrated flow rate measured by a flow sensor in the main flow channel and V.sub.c is the calibrated flow rate. Using C.sub.V is helpful because the correction factor is not dependent of the flow rate, and only depends on the chemical composition and properties of the fluid, primarily the thermal conductivity and specific heat of the fluid.
The flow sensor measuring the uncalibrated flow rate, is typically a thermal anemometer, which measures the difference in temperature between upstream and downstream sensing elements by measuring the differences in resistance between each sensor. Relative temperature changes between the two sensors result from convection effects that occur due to the flow of the fluid passing by the heated elements. From this difference, the raw or "uncalibrated" flow sensor signal and flow rate of a fluid can be derived. If only a single fluid is to be measured, then one could set the C.sub.V =1 assuming the sensor had been initially calibrated to measure this fluid. However, if other fluid compositions are to be measured, then the determination of C.sub.V for these fluids is required.
This known technique of measuring thermal conductivity and specific heat, calculating a correction factor based on these values, and then deriving a normalized flow rate is costly and slow due to the complexity of the data processing involved. The relationship between C.sub.V, specific heat, and thermal conductivity requires substantial computing power to derive the former from the latter two. Slow response time is a particular problem in many applications in which the composition of the fluid may change naturally from minute to minute, a condition that frequently occurs in flow systems in which the fluid is natural gas, gasoline, fuel oil, or other chemical binary, tertiary, or quaternary mixtures.
The present invention is directed to a flow sensor which can be self-normalized to produce accurate flow rate measurements for varying types of fluids, both liquid and gaseous, and to be able to produce such measurements at an affordable cost and with a relatively short response time.